Infectious bursal disease virus (IBDV), a member of the Birnaviridae family, is the causative agent of a highly immunosuppressive disease in young chickens (Kibenge, F. S. B., et al., J. Gen. Virol., 69, 1757-1775 (1988)). Infectious bursal disease (IBD) or Gumboro disease is characterized by the destruction of lymphoid follicles in the bursa of Fabricius. In a fully susceptible chicken flock of 3-6 weeks of age the clinical disease causes severe immunosuppression, and is responsible for losses due to impaired growth, decreased feed efficiency, and death. Susceptible chickens less than 3 weeks old do not exhibit outward clinical signs of the disease but have a marked infection characterized by gross lesions of the bursa.
The virus associated with the symptoms of the disease is called infectious bursal disease virus (IBDV). IBDV is a pathogen of major economic importance to the nation and world's poultry industries. It causes severe immunodeficiency in young chickens by destruction of precursors of antibody-production B cells in the bursa of Fabricius. Immunosuppression causes increased susceptibility to other diseases, and interferes with effective vaccination against Newcastle disease, Marek's disease and infectious bronchitis disease viruses.
There are two known serotypes of IBDV. Serotype I viruses are pathogenic to chickens whereas serotype II viruses infect chickens and turkeys. The infection of turkeys is presently of unknown clinical significance.
IBDV belongs to a group of viruses called Birnaviridae which includes other bisegmented RNA viruses such as infectious pancreatic necrosis virus (fish), tellina virus and oyster virus (bivalve mollusks) and drosophila X virus (fruit fly). These viruses all contain high molecular weight (MW) double-stranded RNA genomes.
The capsid of the IBDV virion consists of several structural proteins. As many as nine structural proteins have been reported but there is evidence that some of these may have a precursor-product relationship (Kibenge, F. S. B., et al., J. Gen. Virol., 69, 1757-1775 (1988)). The designation and molecular weights of the viral proteins (VP) are as shown below.
TABLE 2 ______________________________________ Viral Protein Molecular Weight ______________________________________ VP1 90 kDa VP2 41 kDa VP3 32 kDa VP4 28 kDa VP5 17 kDa ______________________________________
Two segments of double-stranded RNA were identified in the genome of IBDV. The IBDV genome consists of two segments of double-stranded (ds)RNA that vary between 2827 (segment B) to 3261 (segment A) nucleotide base pairs (Mundt, E. et al., Virology, 209, 10-18 (1995)). The larger segment A encodes a polyprotein which is cleaved by autoproteolysis to form mature viral proteins VP2, VP3 and VP4 (Hudson, P. J. et al., Nucleic Acids Res., 14, 5001-5012 (1986)). VP2 and VP3 are the major structural proteins of the virion. VP2 is the major host-protective immunogen of IBDV, and contains the antigenic regions responsible for the induction of neutralizing antibodies (Azad, et al., Virology, 161, 145-152 (1987)). A second open reading frame (ORF), preceding and partially overlapping the polyprotein gene, encodes a protein (VP5) of unknown function that is present in IBDV-infected cells (Mundt, E., et al., J. Gen. Virol., 76, 437-443, (1995)). The smaller segment B encodes VP1, a 90-kDa multifunctional protein with polymerase and capping enzyme activities (Spies, U., et al., Virus Res., 8, 127-140 (1987); Spies, U., et al., J. Gen. Virol., 71, 977-981 (1990)).
It has been demonstrated that the VP2 protein is the major host protective immunogen of IBDV, and that it contains the antigenic region responsible for the induction of neutralizing antibodies. The region containing the neutralization site has been shown to be highly conformation-dependent. The VP3 protein has been considered to be a group-specific antigen because it is recognized by monoclonal antibodies directed against it from strains of both serotype I and II viruses. The VP4 protein appears to be a virus-coded protease that is involved in the processing of a precursor polyprotein of the VP2, VP3 and VP4 proteins.
Although the nucleotide sequences for genome segments A and B of various IBDV strains have been published, it was only recently that the complete 5'- and 3'-noncoding sequences of both segments were determined. The 5'-noncoding region of IBDV segments A and B contain a consensus sequence of 32 nucleotides, whereas the 3'-noncoding terminal sequences of both segments are unrelated, but conserved among IBDV strains of the same serotype (Mundt, E. et al., Virology, 209, 10-18 (1995)). These terminii might contain sequences important in packaging and in the regulation of IBDV gene expression, as demonstrated for other dsRNA containing viruses such as mammalian and plant reoviruses, and rotaviruses (Anzola, et al., Proc. Natl. Acad. Sci. USA, 84, 8301-8305 (1987); Zou, S., et al., Virology, 186, 377-388 (1992); Gorziglia, M. I., et al., Proc. Natl. Acad. Sci. USA, 89, 5784-5788 (1992)).
In recent years, a number of infectious animal RNA viruses have been generated from cloned cDNA using transcripts produced by DNA-dependent RNA polymerase (Boyer, J. C., et al., Virology, 198, 415-426 (1994)). For example poliovirus, a plus-stranded RNA virus; influenza virus, a segmented negative-stranded RNA virus; rabies virus, a non-segmented negative-stranded RNA virus; all were recovered from cloned cDNAs of their respective genomes (van der Werf, S., et al., Proc. Natl. Acad. Sci. USA, 83, 2330-2334 (1986); Enami, M., et al., Proc. Natl. Acad. Sci. USA, 87, 3802-3805 (1990); Schnell, M. J., et al., EMBO J., 13, 4195-4205 (1994)). For reovirus, it was shown that transfection of cells with a combination of SSRNA, dsRNA and in vitro translated reovirus products generated infectious reovirus when complemented with a helper virus from a different serotype (Roner, M. R., et al., Virology, 179, 845-852 (1990)). However, to date, there has been no report of a recovered infectious virus of segmented dsRNA genome from synthetic RNAs only.